The present invention relates to a system for processing recurrent signals to improve the timing quality thereof. More particularly, it relates to digital apparatus for correcting high speed time base errors that often occur, e.g. in transmission or reproduction of television video signals.
Conventional television broadcasting systems create visual images on a cathode ray tube screen through a process of repetitively scanning at a high rate from left to right and from top to bottom. Such television systems typically utilize 525 vertically displaced horizontal lines (scans) for each complete picture called a "frame" and 30 frames are scanned during each second. During the scanning process for each frame, it is important that each succeeding line start and stop in the same horizontal position as the preceding line so that the vertical edges of the generally rectangular picture are straight and so that the reproduced picture itself is a true image of the scene being televised. To synchronize the scan of the cathode ray tube with the transmitted video signal, a horizontal synchronizing pulse is provided at the beginning of each line and a vertical synchronizing pulse at the beginning of each frame during synchronizing intervals.
The processing of television signals in the aforesaid systems often introduces timing errors between successive lines within a video frame called time base errors. These errors are characterized by successive lines being displaced to the right or to the left relative to preceding lines. Thus, the vertical edges of a television tube picture manifesting time base errors are characteristically wavy or ragged rather than being sharp vertical edges as they should be, and the picture is distorted throughout in the same way as the ragged or wavy edges.
A primary source of time base errors in television systems are video tape recorders when they are functioning in the playback mode. Changes in tape dimensions, relative mislocations among reproduction heads in a rotating head assembly, differences between and uneven wear of reproduction heads, and motor velocity errors may singly or in combination introduce time base errors in the video signals being reproduced. In addition, other conditions and devices are known to introduce time base errors into video signals. Whatever the cause of these time base errors, the result was an unacceptably distorted television picture. Thus, a recognized need arose for an apparatus capable of processing video signals to minimize time base errors.
A number of approaches and techniques were developed to eliminate time base errors. All of the approaches heretofore taken were characterized by the substantial drawbacks of complexity, instability and high cost.
One of the techniques known in the prior art utilized a conventional analog lumped-constant delay line of electrically variable length. That technique processed direct video signals to determine the amount of time base error and then electrically lengthened or shortened the delay line to correct the error. The system required continual adjustments and was very expensive. Also, abrupt changes in delay line length could not be made.
A more recent development in the prior art was disclosed in U.S. Pat. No. 3,860,952 to Tallent et al. That system provided very complex and expensive high speed (10.7 MHz average sampling rate) analog-to-digital hardware for digitizing and storing substantial amounts of a video signal at a clocking rate that varied analogously with the time base errors. The digitized video signal was thereafter withdrawn from storage in multiple shift registers at a standard stable rate and regenerated into video signals free of time base errors. That system was unable to make sudden changes in delay correction and was not able to be used with multiple segmented rotary head video tape recorders.
Another proposed system combined digital and analog techniques to provide time base error correction. Input shift registers were to be employed for sequential loading of sampled analog (direct) video information into a bank of metal oxide semiconductor (MOS) storage cells at an uncorrected rate. Thereafter, output registers were to withdraw the analog video signals at a rate free of time base errors. The complications of that system led to difficulties with implementing hardware.
In view of the foregoing a general object of the present invention is to provide a vastly improved and simplified method and apparatus for correcting time base errors in recurrent groups of pulse trains such as lines of video.
Another object of the present invention is to provide a very high speed digital time base error corrector that utilizes inherent propagation delays in digital circuit elements to provide an asynchronous pulse delay line.
A further object of the present invention is to provide a digital time base error corrector that provides random access to various outputs of the digital delay line with a high speed digital counter which counts the amount of time base error of each information group and simultaneously selects the delay line tap required to correct the error counted.
A still further object of the present invention is to provide effective time base error correction of amplitude limited frequency modulated video signals received as playback directly from video tape or disc recorder.
Yet another object of the present invention is to provide a very stable and reliable time base video error corrector that is inexpensive to manufacture, that is stable and reliable throughout extended use, and that requires little or no periodic maintenance or adjustment.